A number of diverse biocompatible materials are known to use in ophthalmological practice, which are based on collagen, i.e., fibrous protein. Collagen serves as a kind of framework performing a supporting function for other proteins, as well as for cells; it is present in all body tissues, in the skin, tendons, and bones.
A variety of techniques of collagen isolation from raw materials are widely known, by its dissolution in acids, alkalis, salts, or with the aid of enzymes, as well as its isolation in a solid undissolved fibrous state by exposing the disintegrated raw material to the effect of salts. The isolated collagen is cleaned from pigments, glycoproteins, and proteoglycans, using the commonly known methods.
One state-of-the-art biocompatible collagen-based material is known to use for making contact lenses (U.S. Pat. No. 4,268,131), said material being in fact a gel based on fibrous collagen or a mixture of fibrous collagen with collagen in a dissolved state.
In order to produce said material diverse methods are used for collagen extraction from raw stock (such as hides, tendons, skin, and others) by means of its dissolution in acids (acetic or citric) or in alkalis, followed by centrifugation of the resultant solution, washing it with water, dehydration, drying, redissolution, filtration, precipitation, and recentrifugation. To produce a lens, a 4- to 10-percent gel is prepared on the basis of collagen in an aqueous acid medium having the pH value of from 2 to 4. Resort is also made to enzymic extraction in the presence of proteolytic enzymes, such as pepsin, trypsin, protease, and others.
However, the aforesaid material cannot be used for making long-lived transplants and those having preset porosity of the material, and hence its gas permeability, since used for producing said material is a protein that features a statistical average mass of from 120 to 130 thousand D (the porosity of the material depends on the geometric dimensions of a molecule and the physicochemical characteristics of collagen by which said material is constituted).
Moreover, the material in question is not resistant to the effect of enzymes as being completely constituted by protein molecules, which are liable to undergo lysis. Thus, low porosity of the material and its low resistance to the effect of enzymes result in reduced biocompatibility of the material with the eve tissues.
Another prior-art collagen-based biological material containing ethylene-unsaturated compounds is known to use for making contact lenses (U.S. Pat. No. 4,388,428).
The material under consideration is in effect a polymerized hydrophilic composition liable to swell in water and consisting of soluble collagen and an ethylene-unsaturated monomer featured by the presence of a polymerizable double carbon-carbon bond.
The material discussed herein is produced by extracting fibrous collagen from animal's hide by enzymic extraction with the aid of pepsin. The thus-extracted and purified collagen is mixed with an aqueous solution of an ethylene-unsaturated monomer, and in the resultant mixture collagen is dissolved by acidifying said mixture with 1.0M HCl till the pH value of 3. The resultant solution is filtered, drawn in a syringe, wherein the solution is degassed, centrifugated and filled into lens moulds.
However, the material dealt with in the form of a hydrogel is featured by inadequate gas permeability and porosity accounted for by geometric dimensions of collagen molecules and of molecules of the monomer used, as well as by a low protein content, low shelf- and service life.